Select Cobb angle from the Measurements drop down menu (or press the C key).
Press the left mouse button and drag the mouse to draw the first segment of the Cobb angle, and then release the button. To draw the second segment, again press the left mouse button and drag the mouse. Release the button to finish drawing the angle.
The Cobb angle value is displayed in degrees.
You can move the segments of the Cobb angle or one of their endings by dragging the segment or the cross-marks with the left mouse button pressed.
The label may also be dragged to a more convenient place if necessary.